In the age of social media the power to share is the power to gather in -- millions of opinions. Which is precisely why the FBI launched what they anticipate to be the largest electronic dragnet in US history. But while going viral can be a force of great good crowd sourcing justice can also spread falsehoods and ruin reputations. As one young man learns the hard way. Here's ABC's John -- Sometimes cliches like being in the wrong place at the wrong time -- devastatingly true. Like near the finish line. For a young man who was photographed with a friend both of them carrying bags. Because yesterday that photograph showed up in the New York Post on the front page labeling them. Bad men sought by the police which came as a complete shock to them. They are kids teenagers. One of whom got the news of his stunning notoriety by a FaceBook where his friends were seeing the pictures. -- -- -- -- And people McFadden I don't know like so many concerns. It was just unbelievable. And terrifying Priscilla. Though is to erase my friends -- -- -- run. And me my retirement that's why he was a face in the crowd on Monday he'd gone to the race not understanding the sign up rules and had hoped to run in the marathon -- He ended up just watching for a wild -- leaving hours before the explosions. It was approximately two hours ago. When I got home trying to -- -- going don't. I didn't want -- right outside troops TV -- -- them. But I'm not. And you can see how may be investigators were circulating photos of people with bags it makes sense. That's how the post says it got a hold of the pitchers. Well here is the -- gets a -- house this morning. -- -- -- I mean that's. Us through -- -- 100000 and what was in it on Monday morning. To talk about it was a running shoes in my stuff the run. Well as of Wednesday with all the stuff coming over his phone he knew he had a problem. Being wrongly accused of an act of terrorism -- its own kind of nightmare ask Richard Jewell who went to the media pile on in public excoriated him. When he was falsely fingered for planting a bomb at the Atlanta Olympics. -- a newcomer to the US have been moved here with his family from Morocco only four years ago didn't know any of that but he's got good instincts and grasped how people could turn on him and they dig -- of his ankle he just -- -- How could you did it. Why would you even do this. He got you so many people he killed. Eight years old -- If you look a little credit. It was a unique another good instinct what he did about it I wanted to please do it when their first city this up. The -- Nicole Diane who looks seventy. He says they talked with him a bit and then sent him home. Glad that it was over with -- he worries now whether this will hurt his chances of getting into college. Which is all he wants that and for this bad memory to go away. John Donvan ABC news Washington.

This transcript has been automatically generated and may not be 100% accurate.

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